LeCroy DDA-125 Stuck in standby (I think)

[TD]

Hello, everone!
This is my first post here. I ran about on hour of searches on the forum looking for answers before post.

 I'm new to all this scope talk, starting classes in 5 weeks and wanted to set myself up with a scope or two.
Anyway, Lecroy DDA-125 was just was dropped off at my door about 8 hours ago. I set it up on the bench for a power up, making sure it was in working condition. It powered it up, booted directly into the main screen. It seemed to be showing a recall screen as I  did not have a probe connected. I power up fast looked great ( screen bright) I adjusted the Position knobs then offset knobs, just a point or two. Checking for activity on the screen. The screen looked great, the unit looks great the Cal sticker are still sealed from two years ago. I walked into the house to print out the user manual. By the time I stepped back into the shop, maybe 5min - 8min at the most.

 The screen was dark (black), The brighter yellow light was now a dimmer green. I select the display button, (nothing), selected Auto setup (nothing), plugged a probe in (nothing). Took a sniffy sniff around for any elect burn smell (nothing). Power off, removed power plug, Power on, the light lit up yellow, snapped into green. Nothing on screen, the power is on. I  don't know much about this unit or scopes, (yet). But it looks to me as if it's stuck in standby mode. I looked at both fans in the rear of the unit only one is spinning. Power off, power on both fans start to spin one stops the second the standby light switches, I hear (faint click) In timing with the light switch, one fan spinning standby light on.

The manual I have said nothing of this situation, Manual does not even really tell you how to pull it out of standby mode. That leads me to believe if you do anything it should pull out of standby. >Any idea's< I will be doing research on this issue. If I find the answer or find out I have to tear her down I will post!  Nice to meet everyone! 

[TD]

If anyone knows where I can find a schematic for this unit or just the PSU of the unit that would be great. I'm used your best buddy (google) so much looking for one. google does not want to cooperate with me any longer. lol! I can't find any schem on any part of this unit. How awesome is that?.

[TD]

Hello, everone!
This is my first post here. I ran about on hour of searches on the forum looking for answers before post.

 I'm new to all this scope talk, starting classes in 5 weeks and wanted to set myself up with a scope or two.
Anyway, Lecroy DDA-125 was just was dropped off at my door about 8 hours ago. I set it up on the bench for a power up, making sure it was in working condition. It powered it up, booted directly into the main screen. It seemed to be showing a recall screen as I  did not have a probe connected. I power up fast looked great ( screen bright) I adjusted the Position knobs then offset knobs, just a point or two. Checking for activity on the screen. The screen looked great, the unit looks great the Cal sticker are still sealed from two years ago. I walked into the house to print out the user manual. By the time I stepped back into the shop, maybe 5min - 8min at the most.

 The screen was dark (black), The brighter yellow light was now a dimmer green. I select the display button, (nothing), selected Auto setup (nothing), plugged a probe in (nothing). Took a sniffy sniff around for any elect burn smell (nothing). Power off, removed power plug, Power on, the light lit up yellow, snapped into green. Nothing on screen, the power is on. I  don't know much about this unit or scopes, (yet). But it looks to me as if it's stuck in standby mode. I looked at both fans in the rear of the unit only one is spinning. Power off, power on both fans start to spin one stops the second the standby light switches, I hear (faint click) In timing with the light switch, one fan spinning standby light on.

The manual I have said nothing of this situation, Manual does not even really tell you how to pull it out of standby mode. That leads me to believe if you do anything it should pull out of standby. >Any idea's< I will be doing research on this issue. If I find the answer or find out I have to tear her down I will post!  Nice to meet everyone!

Ok! so I have stripped it down. I see I have not had any response so I guess I'm posting in the wrong spot or my subject is not interesting enough? Anyway, I have her apart, I have not started to test the power rails yet, that is coming up within the hour. However, I did have some trouble removing the PS for a while there I would have bet that I missed a screw somewhere, but I didn't. The PSU melted the composite material that the plug casing is made out of. It was not a severe meltdown but enough to glue the PSU in place. I"m attaching some pic's of the tear down with some pics of the PSU, mainboard and plug casing. I will keep you post even though no one wants to talk to me. Lmao..lol lol.

[tautech]

I see I have not had any response so I guess I'm posting in the wrong spot or my subject is not interesting enough?

Welcome to the forum.

Fear not, there's a few of us keeping an eye on you....please carry on.
One might expect if this scope hasn't been used for a good while e-caps might have needed reforming but I don't see any, only SMD Tantalums. And while they're much better than dipped bead tants don't discount that one has gone short and shut the PSU down.
Hmmm, that connector....could that be a symptom of failing caps on the mainboard.
Check that PSU first, as it hasn't apparently let any magic smoke out, just check cap values and ESR including the small e-cap for the SMPS IC VCC.
As that connector looks to have supplied current beyond design watch very carefully for any signs of heating anywhere on the mainboard, real careful like. 
Good luck and by all means document this repair step by step.

[nctnico]

AFAIK Lecroy used the same basic design for the LC9000 series and LC5xx / LC6xx series. Yours is likely a LC6xx series oscilloscope with a slightly different front. Schematics can be found at http://www.ko4bb.com/getsimple/index.php?id=manuals. Just look in the Lecroy directory and don't forget to donate. Also be creative when the instrument the service manual is for isn't exactly like yours! From what I have seen the models I listed before all share the same basics so any service manual should at least provide a hint on how the circuit works. BTW It seems the overheating of the power plug is a common problem. Forum member Scopeman has listed several common problems: https://www.eevblog.com/forum/testgear/list-your-test-equipment-score-here!/msg1136236/#msg1136236

[azer]

The scope is basically the same as LC684, but with some special disc drive analysis software, so the service manual on ko4bb should be ok.
Other source of information is the lecroy group on yahoo which also has a private file section.
https://groups.yahoo.com/neo/groups/LeCroy_Owners_Group/info

Some more people with similar problems:
https://groups.io/g/LeCroyOwnersGroup/topic/989565#1653
https://www.eevblog.com/forum/repair/lecroy-lc684dl-displays-random-signals/

The connectors seem to corrode easily which probably adds to the overheating problem. Be sure to clean the connector properly after repair, mine was corroded enough so it didn't boot.
Check the resistance from the burnt connector on the main board to ground if there is a short. The PSU boots stand alone so you can check the voltages separately.

[TD]

I caught on to the fact this scope had most of the same hardware as the LC684 about 3 AM so I downloaded the schematics that I could find on the (684). I have all my that I need as that part goes. had to tear it all the way down to get the PSU out, due to the melting of the plug. (glued it) to the acquisition board. anyway, without running any rail test. I decided to fire up the PSU. It powers up, I felt as if would have all night it I let it. Then I plugged the AUX fan, It stayed up without issue.

 At that time cracked the PSU case open, taking a look at the thermal switch, It looked fine. I just did a quick (?) test, it was open as it should be (I think). I did not look up the switch yet, I'm guessing it's open until the temp threshold then closes putting the unit into sleep mode. I had a long night and busy day. Going out now to test (V) on all the rails. I'LL BE BACK! 

[nctnico]

You need to type upper and lower case exactly as it shows in the captcha picture!

[TD]

Ok, this one tripped me up once before just to double check. I look at the thermostat That was reference a few time, Its a TO-220 Package (Bimetal Disc Thermostat), Its part number is (67F085) I have run into this IC once before. If I remember correctly the part number tells all. 1st two digits =( factory spec or not), letter = (type) and last three = (temp/limit).

If I remember right, "Probable not"    This means, the  67 = (factory set, or basic unit).     The (F) = Fan Unit, (F) = Open On Rise<,   If it was (L) it would be (Limit) >(L) = Close On Rise<

Here is where I'm double checking, with you guys.  It's F so (F) = (Open On Rise) So when the unit is at room temp or under the set Rise temp (it is closed) meaning there is a (closed circuit) right. so if it's not closed at room temp its bad right?  I had a big brain fart on this a few years back now I trip myself up every time I run into this little thermostat unit. LMAO RN I know I'm so over thinking this but better safe than sorry.

[nctnico]

When I put the part number in Google I get this datasheet:
airpax.sensata.com/pdfs/6700.pdf
The F models seem to be open at temperatures under the trip point.

[TD]

When I put the part number in Google I get this datasheet:
airpax.sensata.com/pdfs/6700.pdf
The F models seem to be open at temperatures under the trip point.

I had some time on my hand's so I power up the PSU and tested the (67F085 TO-220 package) and it was closed, after looking on the data sheet the (F) is a (Close On Rise) unit, I had the unit removed from the surface that it was mounted on, so I should not have been getting hot enough to set off the rise, It could be getting hot from too much current. So I checked the volts on the lead leg, (8.1v) @ (0.03A). I know the unit is bad due to the fact it is closed when under the trigger temp. I just wanted to make sure it burn up because of too high of current flowing through it.

I'm probably over thinking this and just should through a new one in.  I thought it might be a good Idea just to make sure the proper voltage is at this point. I attached the schematic I'm just not sure of the voltage within it, the schematic only gives so many voltage reference points. I'm not the best at schematics. This time next year I should be a lot better as I'm starting classes in 5 weeks. I attached the Schematic, pic 07 if you look at pic 03 you can see signs of this getting hot at some point. This is why I just want to be same and make sure the current coming to this point is not high. pic 04 reading with it removed, pic 05 with it back in. same reading. not that it would be different but there are a few components attached to this part of the chassis since the back of the 67f will flow current into the chassis ground the white wire goes to a dead pad on the PCB there are no traces running to the pad of the white wire at least not that I can see and its see through if you put it up to the light.

Thanks, I did run the P# in google,  I have not cleared my history in a long time, I just found a bunch of units for sale, all the way through the 2nd search page. I just cleared my history and BAM! there it was third down from the top. Thank You! funny how google works. I can google something at work, come home run the same search and get completely diff results. It's all in your history, Cookies, search habits. So as a reminder to all, keep your history clear!  or else you look like a moron (like me)

[TD]

Wife & I just got back from a short trip. The part was here, (67F085) TO-220) Thermal sensor, I replaced it, she fired up, the screen came on.  It looked great for about 5-10 sec then, BAMM, POOW FOLLOWED A UPERCUTT.    Same thing happened again!  I did watch a video with same scope model with the same problem. The guy just left it out. I don't feel too comfortable with that unless it's common. Looking for a little help with the Volt / Amp values at this thermal switch. One thing I'm pretty sure of, it's not burning up due to heat from overwork or heat from the mounting plate its mounted to.

 The only thing that comes to mind is, another issue drawing more current that spec, on the same run as the thermal switch. Then the switch is doing its job and popping first.  I have looked over the schematics with a fine tooth magnifying glass     I'm just not sure of the volt amp that should be here. I'm not the best at reading schematics yet, with that said I gave a copy to a friend to look over have not heard anything back in few day's so I guess it did not stick out like a phat thumb. If anyone knows the current values at this switch could you pass it on to me?  Or take a look at the schematic that I posted and let me know. Thanks!  Schematics are attached to my last reply! Thanks Again!

[TD]

Well, I found out the first time I check the Volts there I was getting an incorrect measurement, due to such a tight area and making contact with the wrong post. I did, however, pull the PSU board completely out of the PSU(CASE, then grounded it. Powered it back up, I'm at 6.6V pulling 9.20mA. I'm still not sure on specified voltage and current at this point in the circuit, though?  I can track >J7(the switch) to >J1(Line Input) Does anyone know if my test measurements are within spec or not?   

[TD]

Feeling a little a lost ATM, Really wanted this scope up and running before my (electrical class). Class starts in 4 weeks now.  What I have found out or identified today! If I pull the PSU put it back in it will work for 30 to 60 sec then fall back in the same routine. Once it goes into standby mode (SB) it doesn't matter If I  power off/on, it will do nothing, just boots straight into SB mode. If I Pull PSU put it back in, It will boot just fine for short time.(30-60 sec). I have monitored the temp of the (67F085) along with volts during the boot up until it sinks back into SB mode, Temp 18-19c no higher. This if far from the 85c needed to activate the switch. 

There has got to be another reason for this unit going into (standby) other than the (67F085 thermal switch). I did eventually get the sideboard out of the PSU case and found the numbers to the (resistors) and the (PMOS Transistor) within the same circuit trace as the (67F085). They tested fine as well. I believe the pMOS-transistor is an N type. lack of ex I still get that backward sometimes.  I have to do one more test because I believe the (67F085) is closing but not do to temp. Otherwise the (67F085) just fine out of circuit there is not continuity a crossed it, telling me it's (open) as it should be.

 I'm go start all over starting to get lost. will have to keep better notes. I'm guessing this is a lesson that comes with experience in this field. We know I don't have much of that yet, that's why I'm going to school for the knowledge to gain the EX. Anyway, if anyone has any idea's and/or recommendation on the issue, the problem or test I can run to track this problem down, I'm all ears or eyes in this case but same diff! it would be nice to have my own scope in class then I won't have to wait for others. Thanks again.

[tautech]

PM member Scopeman mentioned in reply #4 and link him to this thread.

[TD]

even though none of the PSU caps show any sign of leaking, bulging, or any other signs of failure.  do you still recommend the desoldering and ESR reading all of them? Also to check the SMD Tantalums on the main board will I have to desolder them as well to check their values?  Also since the power supply output voltages all read as they should, except for the -2 is -2.5 but I don't think that is a sign of short. since the output is good would you disregard the issue being in the power supply and concentrate on the main board? or could the PSU still be outputting proper volt and still have a bad cap or other problems?

[nctnico]

I'd start by going through the schematics of the power supply and see how the thermal switch is connected to the rest of the circuit. What kind of voltages is it connected to and what is the load. I find it odd it is completely fried like that. Just leaving it out is not a good plan.

[TD]

So I dug deeper into the PSU, removed the 120v rail. Once I removed that I could see the big 35x40mm CAP under there. I found a small ceramic cap that was bad, burned right there the board. Now the pads are gone. I removed the resistor that was connected to the same pad as well. I will give a detailed report with pics tomorrow. Have a good night!

[TD]

I Thing I found the problem or at least a good start hiding under the 120V rail, 
 I have found a bad small ceramic CAP and 1 metal film Resistor tied together & one large cap, uF test good but high ESR on the large cap.  I'm having some trouble finding the Exact replacements. especially on the R and small cap.  you can see in the attachments the burnt the board. As I desoldered they just fell apart, even with good heat and fast DE-solder time.  The pressure from my nail start to crumble it with out heat. small CAP will show nothing for capacitance or Ohm on DMM.  So I have posted pic's of the issue and parts. could use some help locating the replacements. I have put a few hours into it with low results.

Large CAP: 450V-330uF-M105C, On Body ( iC LMUM)  35x40MM W-H, I have find close on this one same volt, uF & temp. but size is a little off.

Small CAP:    390pF-  +/- 20%   On Body (391K-1KV-Y5F) Small ceramic cap. green color. having hard time finding this one.

Resistor:   Yellow-White-Black-Gold,  49Ohm I belive metal film resistor. I have find some close but not Exact.

If anyone has good conection of parts & can hook me up with a place to find and or purchase these that would be great. Like I said I can find close. but do to lack of exp with these parts I don't know the trick to the trade yet in finding parts.

[tautech]

Digikey or other big suppliers should have this stuff but another that I've got stuff from Tedss.
www.tedss.com/Capacitors

For the 390pF the value won't be of as much concern as the voltage rating and quite likely 220-680pF will do the job.
It's part of a snubber that may have put under stress as the other components associated with it have drifted over the years and then had to operate outside design spec.

Large cap, physical dimensions can be taller from the pics, the voltage must not be less and the value need be close to the same or a little higher. Too high will increase inrush current and possibly hurt the diode bridge that supplies it.

Link us in to anything you find for confirmation that it will be OK.

[casinada]

Correct me if I'm wrong but that resistor colors look like orange white black gold, that should be 39 ohms 5% resistor. It should be very easy to find almost anywhere. That looks like a 2 Watt resistor. The capacitor is very standard too.
Good luck with your repair. 

[TD]

Correct me if I'm wrong but that resistor colors look like orange white black gold, that should be 39 ohms 5% resistor. It should be very easy to find almost anywhere. That looks like a 2 Watt resistor. The capacitor is very standard too.
Good luck with your repair. 

That would be nice. they are old the white has already come off just from me touching it. 49 Ohm @ 5% is not normal and hard to find.  I was originally looking at it under my florescent light but now that you say something I looked again I believe your right   Thank You (Good looking out!)
I think it measures out to 15mm Long  4mm Thick with calipers. The schematic says's 2. all capacitors are minimum 50 volts.   1. All Resistor values are in Ohms, 5% 1/8W  Unless otherwise specified.  This resistor seems too large for a 1/8W resistor usually a 1/8w resistor is about 4mm long. not all the time but in most cases.

[TD]

Damm:-// I went back out and found some more of the same size and manufacture some gray body. found abut 8 of them all different color codes, most had at least 2 of the same code on the board, but now after finding an orange and more yellow.
Its yellow lucky me. looked at it under the glass and its yellow. at least you got me excited for a few min. I ran the 39 Ohm and found it right away.
I still have no idea of the volt current in this section of the board the schematic is not that informative at least not to me. I'm looking at the LC684DXL schematic as the unit looks to me the same.
The are no volt or amp references on the J1 Rail not that I can find. If someone could take a look I have attached it in the thread. Just to let me know if I'm a moron or not.
 I'm sure someone with more experience then I could look at the board and schematics and figure it right out. but I'm not there

[TD]

thanks found everything @ tedss.com

[tautech]

Correct me if I'm wrong but that resistor colors look like orange white black gold, that should be 39 ohms 5% resistor. It should be very easy to find almost anywhere. That looks like a 2 Watt resistor. The capacitor is very standard too.
Good luck with your repair. 

That would be nice. they are old the white has already come off just from me touching it. 49 Ohm @ 5% is not normal and hard to find.  I was originally looking at it under my florescent light but now that you say something I looked again I believe your right   Thank You (Good looking out!)
I think it measures out to 15mm Long  4mm Thick with calipers. The schematic says's 2. all capacitors are minimum 50 volts.   1. All Resistor values are in Ohms, 5% 1/8W  Unless otherwise specified.  This resistor seems too large for a 1/8W resistor usually a 1/8w resistor is about 4mm long. not all the time but in most cases.

Go with 1 W. (12 x 4mm)

[TD]

Here is another pic of PSU schematic the orange highlights are all Caps, Blue highlights resistors. Red are the (3) I removed, If anyone can tell be the volt at any of those points or rail I would feel more like a moron but better none the less! 

[tautech]

You'd be wise to look hard at all the electrolytics especially those on the secondary side than will mostly be low ESR types and don't forget any ecaps near the SMPS controller. Throwing a ohm meter across the resistors is also a good idea with anyone that reads higher than marked value being suspect.

The RC snubber could be omitted for test purposes but it's there to suppress the noise of the series diode and necessary for an instrument of this class.

BTW, where's the thermal switch ?

[TD]

thermal switch is in green! added to attachment.

[nctnico]

I expect the voltage on the electrolytic in the red square to be around 400V. It is at the output of a PFC stage which usually boosts the mains voltage.

[Rasz]

I expect the voltage on the electrolytic in the red square to be around 400V. It is at the output of a PFC stage which usually boosts the mains voltage.

its also one of the last caps to go bad in a power supply, how high was the high ESR? even couple of ohm should still be "fine"

[TD]

I expect the voltage on the electrolytic in the red square to be around 400V. It is at the output of a PFC stage which usually boosts the mains voltage.

it's also one of the last cops to go bad in a power supply, how high was the high ESR? even couple of ohm should still be "fine"

I have my ESR meter on its way, will be here in a day or two. The large Cap 450v 330uF I just pulled for good measure. I don't know if its bad or not. I do know the small cap is bad and resistor. I just thought I would change it out. but I'm interested in its ESR as well, therefore, I purchased a new meter to be here soon. I'm waiting on the Eyelets for the burnt spot in board anyway so no hurry.

[TD]

The Parts came in yesterday My new ESR meter will be in on Monday. I should be making some progress by Tuesday-Wednesday at the latest.

[TD]

I've attached some pics,
Please look at the pic showing Voltage outputs on 1-11, Pic shows what the schematic states voltage should be. Compared to the actual output of 1-11. 5 are almost right on. However, 3 of them are (Ground) leaving 3 reading (low voltage) and (1) reading (high voltage) as you will see in the attachment. Looking for advice on, whether or not they are far enough out to need adjustment. There are adjustment posts. They are tricky to work with sometimes. Tell me what you think should I raise the 3 up and lower the one, let it be?  Thanks,  now on with the repair that has worked the Oscilloscope is functional.  Yep! WHO'S THE MAN! 

Well, all seems to be good. I never could find the exact 49Ohm 2W 5% resistor, I went with a 49.9 1% 2watt wire wound resistor as that was all I found at the time. I replace the 330pF-450V Cap, 391pF-1k ceramic Cap & 49-Ohm-5% R, Once the Mastech MS5308 was delivered, I ended up replacing a few other caps 1500pF-35V, 470pF-16.3V and a 680pF-50v, along with one other 24Ohm-R.  I went through the board again. Testing with My Fluke 287 and now my new MS5308 LCR Meeter, (I Like It So Far) I did find all 3 of newly found bad Caps listed above to have proper capacitance; However, the ESR was quite high on them. The Capacitance was low but not so low you would call it a bad Cap with just a Capacitance Test. I'm talking the 680pF C-Test was 620pF, but with a 10point high ESR reading.  There were many in series that I had no trouble testing in the circuit. But I ending up pulling 12 for ESR testing. I found (2) bad in the circuit, (1) bad out of the circuit.  I ended up replacing a total of  5 Caps and 2 Resistors.

Note: All bad components that I found in-circuit where pulled and retested. All posted same results with ESR  as they did in-circuit. Within 0.0002 Points! just wanted to through that out in case I was going to take heat for in-circuit testing. I believe it's accurate, Ok to do & a big time saver, depending on how the components are laid out. There were many I did not even waste my time and just pulled as I could see on the layout there is no way you could get an accurate reading. Thanks again! Let me know your thoughts on my voltage outputs and if I should make adjustments, or not. I'm excited to get her up and running and a little proud of my first repair on equipment that I have no experience with. Have a great day! 

[tautech]

Well done, great achievement. 

Enjoy you new/old scope. 

For closure you could post a BOM of parts you replaced, it just might point others to a successful repair too.

[TD]

Well done, great achievement. 

Enjoy you new/old scope. 

For closure you could post a BOM of parts you replaced, it just might point others to a successful repair too.

I was planning on posting a BOM right after I decided on whether I'm going to dial in the output voltage or not? Voltages, Posted in my last post. I'm not sure if those outputs are within spec for this unit or not, was hoping a reader would have some incite on that question. Then I will be sure to Post a BOM, Thanks "Tautech" for helping out with this repair!

[nctnico]

I don't like the -2V being at -2.5V. IIRC -2V is typically used for ECL logic termination so it may be more critical than the other voltages. I'd check what the service manual (of a similar scope) says.

[TD]

I don't like the -2V being at -2.5V. IIRC -2V is typically used for ECL logic termination so it may be more critical than the other voltages. I'd check what the service manual (of a similar scope) says.

Thanks for the reply!  Same here it is the -2V that I was most worried about as well. I dug around for quite some time last night. I can't find a definitive answer on the exceptible voltage for the ECL section of the Pcb. The schematic show it at -2volts, the schematic available online for LC684-LC684DXL and the DDA-125 does not show voltage points within the PCB, only Input-Output voltage. However, I traced it out last night, recording a lot of the voltages at many points in the circuit including Points at semiconductors, ECL Gates, I also check the ECL/TTL gain option of the Oscilloscope, and everything seems ok. But this is coming from someone that is just starting to understand the functions of an Oscilloscope, So I'm not satisfied yet.
I could just adjust that output rail to the -2v, but the old saying "don't go kicking a sleeping dog", or dead horse, or whatever the Hell that saying was..lol,  is weighing on my thought process. I will wait it out a little longer to see if someone has some good advice on the subject or not. Before I go making a decision. Thanks again for your reply!

[TD]

Waiting for my acrylic sheet to get here. I broke the screen cover when installing the front panel. Was only held in with caulk and it just slipped right out onto the floor. Once that is in I will post a BOM.

[azer]

I kinda remember the -2V being -2.5V on my unit also, though I assumed it was down to me not testing it under load with the PSU connected to the main board as required by the manual. I think the manual only specifies the need to adjust +-5V and +-15V anyway.

[TD]

Thanks for the info, I have been on a few other projects Macbook repairs for customers. However, I'm back to working on this Lecroy DDA-125. I'm going to go over it with a fine-tooth comb and see what everything looks like. I plan on making this scope and my Tek784D my two main scopes. I have been working with scopes a lot in class. I'm getting a greater understanding of them. Thanks again!

[Leo Bodnar]

Has the source or alternatives for these connectors ever been established?  I think they were made by LMI for LeCroy.

I have a LC648DXL with similar damage and would rather put something in that would last.

Cheers
Leo

Courtesy TD:

[DaJMasta]

Picked up a DDA-125 sold as not powering up, and while the first few power cycles didn't yield anything, it suddenly came to life after that, and I ran it long enough to verify the printer, some basic acquisition system stuff, the cal signal, all four inputs, etc... seemed to be working great.  Hasn't powered up successfully since then.



So, suspecting the power supply immediately, I set around to take things apart.  Got all the screws out of the power supply and had a lot of trouble getting it to come off - enough that it seemed like there was another screw somewhere.... but eventually the connector gave out and I saw what caused it - almost identical melting of the big power connector.



The PSU's filtering is impressive: AC input goes into an inline RFI filter, then this shielded filter module in the power supply (bottom, with the yellow tape), then into the switcher, then through a massive choke for each rail with bulk caps as well as smaller decoupling caps right on the contacts (in two sizes, no less).



What's more, I opened up the power supply and in addition to taking some measurements of the output voltage, I took some pics with the thermal camera.  Most of the bits heating up quickly were power resistors, but the one part that heated up but wasn't a resistor.... was C93, the same snubber cap that failed in the OP's unit.  Not only that, the only discoloration of the PCB is right under that part too - I had been suspecting it in my preliminary analysis before I found this thread.  This thing may have come from a bad batch.... makes me want to look around the PSU board and just replace all of them or something.



Anyways, I have yet to pull the cap and verify that it was the main issue, but given the intermittent nature of my failure (my latest measurement of the rails actually yields pretty acceptable results, though I've measured way low previously), I wouldn't be surprised that it's gradually coming apart and replacing it will right things.

Otherwise, I plan on swapping the PSU fan with a new one of the current model (the one in their definitely is not living up to it's 29dB noise claim), and swapping the main ducted fan with something with just a hair higher CFM with lower noise.  Then I've got 2x64MB SIMMs on the way to double the system memory on the CPU card, the spec sheet says it can handle up to 128MB modules and the upgrade cost $15.



Took the top shield plate off to get a good look at the acquisition board too - mostly out of curiosity, but to check for damage related to the burned connector too.  31 screws (including the 6 top screws in the plastic BNC mounts) later, it's a nice looking board with a lot of custom parts.  Some high quality bodges to be found, and it looks like the input of the ADCs can be all routed to channel 2 (maybe 3?), verifying why I could get 8GS/s on a single channel when I was testing even though the manual claims you need a PP096 external connector to use all 4 ADCs at once.




Then some fancy bodges on the acquisition board - it's just not a finished product until you have to glue a resistor array to a chip.




Also been really impressed with the metalwork.  You open the top and everything has a nice color and looks clean, then you get inside and it's really well designed, you can see shielding for the front end components as well as some ducting under the shield to make sure the ducted fan in back pulls all the air over the ADCs.

[Leo Bodnar]

Picked up a DDA-125 sold as not powering up...

...and it looks like the input of the ADCs can be all routed to channel 2 (maybe 3?), verifying why I could get 8GS/s on a single channel when I was testing even though the manual claims you need a PP096 external connector to use all 4 ADCs at once.

Very odd, this acquisition board looks like DDA-120 / LC574 series.  Both front end and add-on channel multiplexer/combiner.
Can you verify its frontend bandwidth or risetime?

Did you get it from motion_constrained?

Leo

One of my DDA-125 frontends (I have replaced a few relays):

[DaJMasta]

Yep, one of the two they recently sold.  From the board number on that front end amp to ADC routing board of sorts, my part number is one lower but two revisions later.

I didn't run particular tests to verify that it was actually sampling at 8GS/s, but on channel two in single channel mode, it was reporting 8GS/s, and since there's an eeprom in those little bridging adapters, I figured it would know if only one channel was actually getting the signal.

[Leo Bodnar]

I have checked the other one they have sold (I got it) and it has the standard acq board (like the one I have posted.)
You got yourself an interesting instrument.
I know of someone who thinks that DDA-120 with LCD would have been the best instrument from LC series and it looks like it exists and you have it.
Try checking if it's really 1.5GHz BW or 1.0GHz like it would be on DDA-120.  This would be the only downside and possibly caused by complicated channel multiplexer/combiner - that add on mezzanine board.  On DDA-125 it is much simpler and does not have any active parts.
Leo

[DaJMasta]

Interesting!  It's SN 30005, manufactured on the 7th of September, 1999, if that helps anyone get an idea of when such a change may have been made.  I wonder if there are LC684s that are the same way.

[DaJMasta]

A little update on my repair:

I ended up pulling C93, but while it was discolored and the PCB was singed, it measured alright.  Replaced with a 500pF 1kV cap I had in a box, which read a little under so it should technically still be at the edge of the 20% tolerance of the original part.  I haven't tried powering up the full thing, but most of that is because of a strange happening.

In measuring the outputs again, everything seems to be in spec except for the -15V rail.  As it turns out, the spec is VERY broad, so even 5.8V on a 6V rail is actually still 200mV above minimum spec, and that -2.5V "-2V" rail is still within the -2.6V maximum (minimum?) spec.  However, the -15V rail was reading high, often under -16V.  So I went to adjust the trimpot and found that not only did it make almost no difference, but the potential was gradually rising... it got to under -17V by the time I powered it down after trying to adjust it.

I also found that those bigger resistors on the output are probably actually supposed to run hot... looks like at the -15V spec, R2, which is in parallel with the -15V rail, dissipates about a watt.  I guess to keep noise down they've got a constant resistive load right on the output.  Now there's a chance when the rail is more heavily loaded it will be pulled down to where it should be, but with the constant load and no response on the adjustment trimpot, I think the rail regulation just isn't working.

Checked BR3, the rectifier for the rail, it was fine.  Checked Q7 and at least on the diode check it matched the other transistors next to it on functional rails... so I don't think it's that.  But there's only so many active parts just on this rail... so maybe the current loading measurement amp on the main board, U1, does not have a high impedance input, or maybe the LM358, U3 on the housekeeping board, is a dead amp and just isn't driving the adjustment transistor at all?

It's hard to say, maybe have to probe around with a scope or something.  If I can't verify a failure, I may just try to power the thing up and see, but it's unsettling seeing a rail with active regulation gradually drifting up - maybe the regulation is stuck and it's increasing resistance of the load resistor as it heats?



Also, it's real annoying working on the power supply out of its chassis because of the one heatsink plate that's covered in thermal paste.  Got some on my fingers many times already, but not so many times that I want to wipe it off and reapply.... at least not yet.

[DaJMasta]

Probed around, removed a couple more parts... but couldn't find another failure.  Applied a 50 ohm load to the -15V rail and suddenly it was being regulated again, so I mostly assembled it (leaving the power supply top off for thermal camera work), and turned it on - and it powered up.  Got my thermal pictures and the resistors were still getting real hot, but it didn't report any errors or anything.  Powered off, powered on, doesn't come up - the main fan also turns on briefly and drops, so I figure it's time to check the fan power supply.

The -12V rail was reading something like -13.2V, so I adjusted it down and hooked up the fan to verify it could run loaded - it did.

So there are a couple of directions to go, I think I need to adjust the -15V line again - while it held regulation, I think it was to -15.6, which is technically out of spec, so I figure it's worth tweaking down.  It could also be worth desoldering the thermal switch and trying to see if it's not opening properly - I think I measured a 2k resistance across when powered down, in circuit.

Otherwise, I want to monitor the shutdown signals going to the power supply from the other board connector.  If those above aren't doing it, there's a chance there's some sort of malfunction on the acquisition board's monitoring circuits that wasn't visible damage.

As for the cooked connector, I used an xacto knife to make sure the opening was as wide as the others and tried to shape up the sides of the melted bits a little, I also sprayed them all with contact cleaner (PSU side).  It feels like each pair of contacts on the PSU side is a separate bit of plastic held in by the big housing, and the metal contacts themselves are soldered directly to the board, so if it turns out that the connector is near impossible to find, finding the little contact pairs may still be possible.  Don't think I'll swap mine because it does seem to power up at least sometimes fine, but who knows.  Not yet sure what's causing my intermittent failure.

Attached the image from the thermal camera of the back half or so of the PSU, installed in the successfully powered up unit, and you can see the temps we're seeing after only 45 seconds or so.  Thermals should be better when the supply is ducted properly with its top installed, but they're still running hotter than I'd like.  The bright one on the right is R12 on the housekeeping board and for the life of me I can't figure out why it's running so hot.  The ones in the center are fixed loads on the rails and are right in front of the fan, so i figure they are intentional, but R12 maybe isn't.  I also think that C93 itself probably isn't prone to failure.... it's just next to two resistors that heat up and gradually cook it.  A 2W on one side, a 5W on the other, and both get up there somewhat.

[IanMacdonald]

Not sure if you solved the resistor value issue, but it's likely a 47R with a missing purple band.

[DaJMasta]

So I think I've found it.

Checked the thermal switch, monitored the shutdown lines, tweaked the -15V rail, monitored voltages from the processor card when trying to power up... nothing.  But, sometimes I could get a hint of a partial startup still, so I started wiggling the power supply.  Pushing to the sides or front/back didn't do it.... but pressing down on the fan in the back got it to instantly boot.  The problem is the connector, and I think it's the same problem that caused the melting initially.


My theory, is that the contacts just don't make good contact... or the board that's giving them rigidity (though properly supported with screws to the chassis all around the connectors) has flexed with age.  This means that the melted contact probably wasn't a surge condition... it was just a sort of weak pressure point with too high contact resistance trying to pass several amps.  The trick is, I'm not sure how to get the connector housing off to clean/fix this on the PSU side, so I think my potential options would be to jam shims in (seems like a bad idea, honestly), or to try and tin the contacts on the acquisition board to make a little more height and thickness (though, probably less even contact and then there's durability issues).  So I don't really know what the best route forward is.

I will try to get the connector housing off and take a look at the bits on the PSU side, but we'll see how far that gets me.

In other news, it's this scope's 18th birthday tomorrow.

[Converter]

Just do it the same as LeCroy does. This will slightly complicate the disassembly, but there is free space and you can add a new connector to the middle of the conductor. I recently repaired my DDA-120 power supply, I was too lazy to add another connector - I just cut this wire, and then connected it again with soldering.

[DaJMasta]

Hahaha, that's LeCroy's official solution?

While it's perfectly reasonable to do... somehow I feel bad about messing up the original drop-in modular design, even if it ended up not being great in the long run, it seemed like a nice solution 


I may try just tinning the socket connectors first just to see if I can get around jumpering them, but we'll see if it does anything.  As it stands, with maybe a 20-30 pound weight on the top of the power supply, I think it would boot up quite repeatedly, though this may just exacerbate the problem with time.  It's odd though, looking at the contacts on the acquisition board side, it seems like most electrical contact is being made on the side of each blade, in a circular pattern over the fork gap.  I think the contacts on the PSU side have been sort of snapped in over little plastic bumps which make both the contact point and the locking mechanism to hold on the connector, but since the connector is melted, I don't think there's a way to get it off non-destructively anymore aside from desoldering everything from the main board.

[Leo Bodnar]

I think the contacts on the PSU side have been sort of snapped in over little plastic bumps which make both the contact point and the locking mechanism to hold on the connector, but since the connector is melted, I don't think there's a way to get it off non-destructively anymore aside from desoldering everything from the main board.

You'll find that plastic is very brittle and will crack easily when you try removing it. 
And when you get to the PSU connectors you'll find blade terminals just like on acq board with measly clips around them.

I have used DIN 41612 Type H11 female socket connectors for repair. 
Opened them up, extracted the terminals and soldered them to cut off stubs on the PSU.

Doing it right needs complete replacement of both connectors.

Leo

[Converter]

Hahaha, that's LeCroy's official solution?

Do you doubt this? It's normal practice for LeCroy to add a bunch of new wires to their devices
Previously, I showed a photo of my DDA-120. And here you will see the same thing in the photo of the other guy: Click
And also for example Here
Since the added wires on the LeCroy board already have so many, it does not look unusual

[DaJMasta]

Haha, I didn't but I still think it's funny.

Bending over the blades on the bottom terminals for extra strength is a nice touch.  I'll make my attempt this evening, and maybe I'll end up with another example of a nice bodge.

[DaJMasta]

I gave tinning a shot, and it seems to have done the trick.  I ended up removing a bit of solder after this pic, and then did a little extra carving of the damaged housing in the top, as the tolerance for the contacts actually seems to be fairly tight.  Especially on the pins that had seen heat damage, it was tough to get the silver solder I used to stick, but an upgrade to the flux I was using did a decent job.  I've got maybe 8 or so boots without a failure so far, but I think it's good to wait and see a bit, since it's a sort of problem that could worm its way back.



Anyways, hooked it up to the RF gen to check whether it was really running 8GS/s and what the -3dB point really was.  The 8GS/s mode is available on any channel, to my surprise, that special mezzanine board seems to be full-featured.  Not only that, but with the statistics on, I measure just a hair over 3dB of loss with the peak to peak measurement, though the RMS is not accurate down in this range.  In a cursory glance through things, does this scope have a sin(x)/x interpolation mode that can run with the statistics?



Also worth noting that the lower volts per division modes really attenuate the signal (it does say this in the datasheet), having some initial trigger trouble, I zoomed in to get a good lock, but read about a tenth of the peak to peak value of the signal.  Zooming out got it back, but I was a little surprised the first time.  That trigger trouble was actually caused by using the internal timebase, it seems not so stable when you're only getting a handful of samples per period, when I switched in the same external reference as the RF gen, it was just fine.

Happy Birthday, DDA-125!

[Converter]

Congratulations, DaJMasta.
But, I think that you could turn on the sweep mode in your oscillator and the Oscilloscope FFT would show you the frequency response more representative.
I used a noise generator to test my DDA-120. The result is shown in the photo.
Frequency response for modes less than 10 mV/div. in fact, greatly worsens. It seems, this is said in the datasheet. This is true for all digital oscilloscopes.

[DaJMasta]

Getting more familiar with the interface, sin(x)/x interpolation is really easy, I just hadn't really considered the way this scope is setup in sort of functional layers.  Similar settings to above, I fed a +5dBm sweep from 1MHz to 3GHz into the scope and took some FFTs.  You could see some higher harmonic tones well beyond 3GHz, was a bit surprised to see them showing up with an 8 bit digitizer, but they were certainly visible.  Looks like the filtering is setup as advertised, where that first little knee is right about 1.5GHz (though if I'm interpreting it right, it's reading 1.5GHz as closer to a -5dBm point), but you can still get usable input to 2GHz if you're just looking for signal presence.  There is some signal response beyond there, but it would be so small that it would be tricky to visualize normally with the bandwidth restrictions at very low levels - though i suppose an active probe would overcome that.  And remember, this is the response with the 'built-in PP096'.



It's a neat (not so) little beast, I feel like the UI takes a lot of 'keystrokes' to navigate, but things are setup pretty well.  I think the biggest complaint so far is that they had to shorten measurement names to only a few characters, so trying to guess which 5 letters they used for which measurement has been fun 

[Converter]

Thus, we can say that your frequency response DDA-125 does not differ from my DDA-120 .
Perhaps this is due to the presence of your multiplexer, as in DDA-120, which should not be in DDA-125.

[Enginerding]

Working through an intermittent PS problem exactly like DaJMasta's.  W/ an unloaded PS, C93 (ceramic, 390pF, 1kV), gets up to ~140F in about 20 seconds (thermal cam).

Pulled C93 and tested it with the DE-5000 LCR @120Hz = 401pF, D 0.021, Q 59.4, ESR 12K-300K, -89 phase angle.  The ESR and Q ar alllll over the place.

I grabbed a new (caveat: new from Aliexpress) 470pF, 1kv ceramic.  I tested @120hz = 470pF, D 0.003, Q 135,  ESR 20k-50k, -89.5 phase angle.  ESR also all over the place.  This has to be some kind of measurement error, right?

I've pretty comfortable with using ESR to grade electrolytics, but I've never looked at ceramic caps in the pF range.  I'm stumped by this measurement.